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Tuesday, 26 June 2012

String Phenomenology: Day 1

Here is a brief overview of the more interesting points of day 1. I
will link to the abstracts of each talk; hopefully, in time, the same
pages will also include the slides and video from the talks.

Ben
Allanach kicked things off by describing some of the lates experimental
results, and what they might mean for supersymmetry (SUSY) in particular (and
hence for string model building, basically all of which is supersymmetric).
His most important points (I think) were the following:

Discovery of a standard-model-like Higgs, with a mass of around
125 GeV, could be
just
around the corner. In many popular realisations of SUSY breaking, a Higgs
mass of 125 GeV is right at, or just beyond, the maximum possible value,
assuming that superpartner masses are kept below several TeV.

There is an unexplained anomaly in the Tevatron data, in the
'forward-backward asymmetry' in the production of top-anti-top pairs.
The Tevatron collided protons and anti-protons, and this variable measures
the number of tops which are produced travelling in the same direction as
the initial proton, compared to the number travelling in the direction of
the anti-proton. The measured value disagrees with the standard model
prediction by something like $3\sigma$.

LHCb is beginning to measure certain rare meson decays for the first time.
Ben talked in particular about $B_s \to \mu^+\,\mu^-$, which can be
particularly sensitive to new physics contributions (since a bottom
quark must annihilate with an anti-strange-quark, this is an interaction
involving a 'flavour-changing neutral current'; these are highly
suppressed in the standard model by the
GIM mechanism).

The Daya Bay neutrino oscillation experiment recently made the first
conclusive measurement showing that the neutrino mixing parameter
$\theta_{13}$ is non-zero. This rules out, in particular, the
popular tri-bi-maximal mixing scenario for neutrinos. To be honest,
very few, if any, string models can make such precise predictions
anyway, so although this is important, I doubt the audience here feels
one way or the other about it.

Ben also has a couple of recent papers exploring R-parity-violating supersymmetry.
This can apparently explain the forward-backward asymmetry, as well as
effectively 'hiding' the superpartners for longer at the LHC, due to the lack
of the classic missing energy signatures of supersymmetry with R-parity.

Bobby
Acharya then talked about his work on 'generic' predictions from string
theory, which I mentioned
here.
Their reasoning seems to start with the following argument:

In spontaneously-broken supergravity theories with moduli (scalars which
are massless in the supersymmetric limit, and have Planck-suppressed
couplings), the moduli cannot all be made significantly heavier than
the gravitino.

All moduli must be heavier than about 30 TeV, otherwise they decay during
or after big bang nucleosynthesis, and hence ruin its successful
predictions. By the above point, the gravitino mass must therefore be at least
30 TeV. The gravitino mass is roughly
$m_{3/2} \sim \frac{M_{\text{SUSY}}^2}{M_{\text{Pl}}}$, where
$M_{\text{SUSY}}$ is the fundamental SUSY-breaking scale.
So $M_{\text{SUSY}}$ is bounded by this argument.

The picture which apparently follows from the above is that sleptons and
squarks have masses of tens of TeV, leaving only gluinos, charginos, and
neutralinos at LHC-accessible energies.

The only clear way out of the above argument seems to be to have a late
period of low-scale inflation, which dilutes the moduli and prevents them
from causing problems with cosmology. However, Fernando Quevedo also
seemed to think that the first point above may not be completely
watertight. The overall reaction to the claims of Acharya et al
seems to be scepticism, but I haven't heard any concrete arguments
against them yet. Gordy Kane will be talking more about this on day two.